这边博客连接上一篇 mybatis 的 xml 解析的博客，在 xml 解析实现之后，首先会解析成一个 Configuration 对象，而后创立一个 DefaultSqlSessionFactory 的 session 工厂。在这所有的筹备过程实现之后，就能够开始对数据库的操作了。
首先看 openSession() 办法
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause:" + e, e);
} finally {ErrorContext.instance().reset();}
}首先，依据 configuration 中取出的 environment，而后获取一个 TransactionFactory，接着通过事务工厂新建一个事务对象，其实在这一个步骤，并没有对数据库进行操作 newTransaction 办法仅仅是返回了一个 Transaction 对象，这个对象蕴含了 Datasource, level,autocommit 这几个属性，并没有做其余操作。（这里我 xml 中配置了 JDBC 事务，具体看这个事务，而不是第三方的事务）。
接下来创立一个 Executor 对象
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {executor = new ReuseExecutor(this, transaction);
} else {executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}以后必定是创立了一个默认的 Executor，就是 SimpleExecutor，而后往下，判断是否配置了开启缓存，是的话则通过装璜器模式创立一个 CachingExecutor，接着调用 interceptorChain.pluginAll 办法返回一个被层层代理的对象，这部分在上一篇博客中剖析过。返回 executor 对象，再接下来，new 了一个 DefaultSqlSession 再返回，至此 openSession 办法就执行完结了。接下来咱们就能够调用 DefaultSqlSession 的 select 或者 update 等办法操作数据库了，不过还是看比拟支流的办法。
BlogMapper mapper = session.getMapper(BlogMapper.class);
Blog blog = mapper.selectBlog(101);首先看 getMapper 办法，调用的是 configuration 对象中 mapRegister 的 getMapper 办法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {throw new BindingException("Type" + type + "is not known to the MapperRegistry.");
}
try {return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause:" + e, e);
}
}所以在上一篇博客中看到哪个 addMapper 办法，寄存的是一个 MapperProxyFactory 工厂，就是因为这里每次 getMapper 会从对应的工厂中创立代理，这里是 Proxy 动静代理
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface}, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}先返回，此时曾经获取到了 BlogMapper 的代理对象，而后执行 selectBlog 办法，这时候会执行到之前的代理办法中，找到之前的
MapperProxy 类的 invoke 办法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {if (Object.class.equals(method.getDeclaringClass())) {return method.invoke(this, args);
} else {return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
}
} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);
}
}这里必定不是 Object 类，所以执行 cachedInvoker()
private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
try {
return methodCache.computeIfAbsent(method, m -> {if (m.isDefault()) {
try {if (privateLookupInMethod == null) {return new DefaultMethodInvoker(getMethodHandleJava8(method));
} else {return new DefaultMethodInvoker(getMethodHandleJava9(method));
}
} catch (IllegalAccessException | InstantiationException | InvocationTargetException
| NoSuchMethodException e) {throw new RuntimeException(e);
}
} else {return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
});
} catch (RuntimeException re) {Throwable cause = re.getCause();
throw cause == null ? re : cause;
}
}computeIfAbsent，这是 jdk8 的语法，大略就是看 map 中有没有这个 key，没有就新建一个并返回新建的这个，有就间接返回，所以这里就是对办法会做一个缓存。当初是第一次执行，必定是没有，所以会执行前面的创立办法。m.isDefault 这些是兼容 jdk8 以上的接口的默认办法，实现是间接运行那个默认办法。
间接看 PlainMethodInvoker，进入 new MapperMethod
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}先看 SqlCommand 的创立
public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {final String methodName = method.getName();
final Class<?> declaringClass = method.getDeclaringClass();
MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
configuration);
if (ms == null) {if (method.getAnnotation(Flush.class) != null) {
name = null;
type = SqlCommandType.FLUSH;
} else {throw new BindingException("Invalid bound statement (not found):"
+ mapperInterface.getName() + "." + methodName);
}
} else {name = ms.getId();
type = ms.getSqlCommandType();
if (type == SqlCommandType.UNKNOWN) {throw new BindingException("Unknown execution method for:" + name);
}
}
}首先，先从 configuration 中查找出对应的 MappedStatement，查找的过程是这样的，先查看以后的类是否存在对应的 MappedStatement，如果有间接返回，否则从父类中查找是否有对应的 MappedStatement
private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
Class<?> declaringClass, Configuration configuration) {String statementId = mapperInterface.getName() + "." + methodName;
if (configuration.hasStatement(statementId)) {return configuration.getMappedStatement(statementId);
} else if (mapperInterface.equals(declaringClass)) {return null;}
for (Class<?> superInterface : mapperInterface.getInterfaces()) {if (declaringClass.isAssignableFrom(superInterface)) {
MappedStatement ms = resolveMappedStatement(superInterface, methodName,
declaringClass, configuration);
if (ms != null) {return ms;}
}
}
return null;
}
}SqlCommand 创立实现之后，再看 MethodSignature 的创立
public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
if (resolvedReturnType instanceof Class<?>) {this.returnType = (Class<?>) resolvedReturnType;
} else if (resolvedReturnType instanceof ParameterizedType) {this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();} else {this.returnType = method.getReturnType();
}
// 返回类型是否是 void
this.returnsVoid = void.class.equals(this.returnType);
// 返回类型是否是汇合
this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
// 返回类型是否是游标
this.returnsCursor = Cursor.class.equals(this.returnType);
// 返回类型是否是 Optional
this.returnsOptional = Optional.class.equals(this.returnType);
// 如果有 @Mapkey, 返回 mapKey
this.mapKey = getMapKey(method);
// 是否是 Map
this.returnsMap = this.mapKey != null;
// 找到第几个参数是 RowBounds
this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
// 找到第几个参数是 ResultHandler
this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
// 参数解析器, 解析 @Param 中的名称
this.paramNameResolver = new ParamNameResolver(configuration, method);
}创立实现之后，返回，调用 PlainMethodInvoker 的 invoke 办法，办法中调用的是 mapperMethod 的 execute
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {result = executeForCursor(sqlSession, args);
} else {Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for:" + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method'" + command.getName()
+ "attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}这里看 select 办法
result = sqlSession.selectOne(command.getName(), param);@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.selectList(statement, parameter);
if (list.size() == 1) {return list.get(0);
} else if (list.size() > 1) {throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found:" + list.size());
} else {return null;}
}public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {throw ExceptionFactory.wrapException("Error querying database. Cause:" + e, e);
} finally {ErrorContext.instance().reset();}
}在这里调用 executor 的 query 办法，因为这里是个装璜者对象，所以看 CachingExecutor 的 query 办法
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {Cache cache = ms.getCache();
if (cache != null) {flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {ensureNoOutParams(ms, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}首先从 MappedStatement 中获取 Cache，这个 Cache 在解析 xml 的时候就曾经创立了，如果获取到的不是 null，那么首先执行 flushCacheIfRequired，这个是通过在解析 xml 的时候判断是不是 select 决定的，除了 select 语句都是 true, 执行革除缓存，接下来从缓存中获取，如果有缓存，间接返回，如果没有，就执行查问。
接着看委托类的实现，这个委托类的实现在 BaseExecutor 中
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {clearLocalCache();
}
List<E> list;
try {
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {queryStack--;}
if (queryStack == 0) {for (DeferredLoad deferredLoad : deferredLoads) {deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();}
}
return list;
}这里又是一个 Cache, 不过这个 Cache 是 mybatis 内置的 Cache，这就是常说的一级缓存，而这个一级缓存的革除，从代码上看，首先是配置了 LocalCacheScope 是 STATEMENT 的时候，默认是 Session，而后就是当执行了 close 办法的时候。
再接着往下看，如果没有命中缓存，就会继续执行查询方法
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {localCache.removeObject(key);
}
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {localOutputParameterCache.putObject(key, parameter);
}
return list;
}@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {Configuration configuration = ms.getConfiguration();
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {closeStatement(stmt);
}
}doQuery 办法是实现类中的办法，以后是 SimpleExecutor，先看 StatementHandler 的获取
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {switch (ms.getStatementType()) {
case STATEMENT:
delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case PREPARED:
delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
case CALLABLE:
delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
break;
default:
throw new ExecutorException("Unknown statement type:" + ms.getStatementType());
}
}首先看 RoutingStatementHandler，是通过不同的 StatementType 创立不同的 Handler 处理器，MappedStatement 新建默认是 PREPARED,CALLABLE 是存储过程，STATEMENT 就不说了，所以失常状况下创立的都是 PreparedStatementHandler，进入构造方法
public PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);
}protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}重点是最初两行，参数处理器和后果处理器的创立
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
ResultHandler resultHandler, BoundSql boundSql) {ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
return resultSetHandler;
}创立之后，两者都有一个操作，就是应用 interceptorChain.pluginAll 进行了包装代理，
返回到 newStatementHandler，interceptorChain.pluginAll 对 RoutingStatementHandler 同样做了一个包装代理, 持续返回
再往下看 prepareStatement 办法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt);
return stmt;
}首先获取一个连贯，接着执行 handler 的 prepare 办法，办法中调用的是委托类也就是 PrepareStatementHandler 的 prepare 办法，其是由父类实现的
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {statement = instantiateStatement(connection);
setStatementTimeout(statement, transactionTimeout);
setFetchSize(statement);
return statement;
} catch (SQLException e) {closeStatement(statement);
throw e;
} catch (Exception e) {closeStatement(statement);
throw new ExecutorException("Error preparing statement. Cause:" + e, e);
}
}这就是创立了一个 JDBC 的 statement, 接下来返回继续执行 parameterize 办法
public void parameterize(Statement statement) throws SQLException {parameterHandler.setParameters((PreparedStatement) statement);
}能够看到调用的是之前创立的 ParameterHandler 的 setParameters 办法，把参数设置到 statement 中，这里须要留神的是，尽管 ParameterHandler 被 plugins 代理比 RoutingStatementHandler 晚，然而实际上 ParameterHandler 办法的调用是在前面，所以拦挡的程序也在前面。
再往下看，接着会调用 query 办法
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {PreparedStatement ps = (PreparedStatement) statement;
ps.execute();
return resultSetHandler.handleResultSets(ps);
}这里就是 JDBC 的执行了，最初看 ResultSetHandler 执行的 handleResultSets 办法
public List<Object> handleResultSets(Statement stmt) throws SQLException {ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<>();
int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt);
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount);
while (rsw != null && resultMapCount > resultSetCount) {ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rsw, resultMap, multipleResults, null);
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {while (rsw != null && resultSetCount < resultSets.length) {ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}
return collapseSingleResultList(multipleResults);
}这段代码大略就是获取出 resultsmap，而后对后果进行解析，所以重点是 handeResultSet 办法
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {if (parentMapping != null) {handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {if (resultHandler == null) {DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null);
multipleResults.add(defaultResultHandler.getResultList());
} else {handleRowValues(rsw, resultMap, resultHandler, rowBounds, null);
}
}
} finally {// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}首先判断 parentMapping 是否 null，null 就是最外层的 resultMap, 而后判断是否有自定义的 resultHandler，有的话用自定义的，没有就用默认的，解析实现之后返回，一路回到 SimpleExecutor 中，最初会执行 closeStatement 办法敞开连贯。
返回后果，执行完结，mybatis 的执行流程也就完结了。
关注公众号:java 宝典